1. Field
The present invention relates to point-to-multipoint services, in a wireline or a wireless communication system. More specifically, the present invention relates to a method and an apparatus for a power control in such a point-to-multipoint services communications system.
2. Background
Communication systems have been developed to allow transmission of information signals from an origination station to a physically distinct destination station. In transmitting information signal from the origination station over a communication channel, the information signal is first converted into a form suitable for efficient transmission over the communication channel. Conversion, or modulation, of the information signal involves varying a parameter of a carrier wave in accordance with the information signal in such a way that the spectrum of the resulting modulated carrier is confined within the communication channel bandwidth. At the destination station the original information signal is replicated from the modulated carrier wave received over the communication channel. Such a replication is generally achieved by using an inverse of the modulation process employed by the origination station.
Modulation also facilitates multiple-access, i.e., simultaneous transmission and/or reception, of several signals over a common communication channel. Multiple-access communication systems often include a plurality of remote subscriber terminals requiring intermittent service of relatively short duration rather than continuous access to the common communication channel. Several multiple-access techniques are known in the art, such as time division multiple-access (TDMA), frequency division multiple-access (FDMA), and amplitude modulation multiple-access (AM). Another type of a multiple-access technique is a code division multiple-access (CDMA) spread spectrum system that conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wide-Band Spread Spectrum Cellular System,” hereinafter referred to as the IS-95 standard. The use of CDMA techniques in a multiple-access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE-ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention.
A multiple-access communication system may be a wireless or wire-line and may carry voice and/or data. An example of a communication system carrying both voice and data is a system in accordance with the IS-95 standard, which specifies transmitting voice and data over the communication channel. A method for transmitting data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION”, assigned to the assignee of the present invention. In accordance with the IS-95 standard, the data or voice is partitioned into code channel frames that are 20 milliseconds wide with data rates as high as 14.4 Kbps. Additional examples of a communication systems carrying both voice and data comprise communication systems conforming to the “3rd Generation Partnership Project” (3GPP), embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), or “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems” (the IS-2000 standard).
In a multiple-access communication system, communications between users are conducted through one or more base stations. A first user on one subscriber station communicates to a second user on a second subscriber station by transmitting data on a reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on a forward link of the same base station, or the other base station, to the second subscriber station. Likewise, the communication can be conducted between a first user on a mobile subscriber station and a second user on a landline station. A base station receives the data from the user on a reverse link, and routes the data through a public switched telephone network (PSTN) to the second user.
In a wireless communication system, maximizing a capacity of the communication system in terms of the number of simultaneous telephone calls that can be handled is extremely important. The capacity in a spread spectrum communication system can be maximized if the transmission power of each subscriber station is controlled such that each transmitted signal arrives at a base station receiver at the same signal level. However, if a signal transmitted by a subscriber station arrives at the base station receiver at a power level that is too low, quality communications cannot be achieved due to interference from the other subscriber stations. On the other hand, if the subscriber station transmitted signal is at a power level that is too high when received at the base station, communication with this particular subscriber station is acceptable but this high power signal acts as interference to other subscriber stations. This interference may adversely affect communications with other subscriber stations. Therefore, each subscriber station needs to transmit the minimum signal level expressed as e.g., a signal-to-noise ratio, that allows transmitted data recovery.
Consequently, the transmission power of each subscriber station within the coverage area of a base station is controlled by the base station to produce the same nominal received signal power at the base station. In an ideal case, the total signal power received at the base station is equal to the nominal power received from each subscriber station multiplied by the number of subscriber stations transmitting within the coverage area of the base station plus the power received at the base station from subscriber stations in the coverage area of neighboring base stations.
The received power is determined by an attenuation of the transmitted power by a path loss of the link. The path loss can be characterized by two separate phenomena: average path loss and fading. In many communication systems, e.g., IS-95, W-CDMA, IS-2000, the forward link and the reverse link are allocated separate frequencies, i.e., the forward link operates on a different frequency than the reverse link. However, because the forward link and reverse link frequencies are within the same general frequency band, a significant correlation between the average path losses of the two links exists. On the other hand, fading is an independent phenomenon for the forward link and reverse link and varies as a function of time.
In an exemplary CDMA system, each subscriber station estimates the path loss of the forward link based on the total power at the input to the subscriber station. The total power is the sum of the power from all base stations operating on the same frequency assignment as perceived by the subscriber station. From the estimate of the average forward link path loss, the subscriber station sets the transmit level of the reverse link signal. Should the reverse link channel for one subscriber station suddenly improve compared to the forward link channel for the same subscriber station due to independent fading of the two channels, the signal as received at the base station from this subscriber station would increase in power. This increase in power causes additional interference to all signals sharing the same frequency assignment. Thus a rapid response of the subscriber station transmit power to the sudden improvement in the channel would improve system performance. Therefore, it is necessary to have the base station continually contribute to the power control mechanism of the subscriber station.
Thus, the subscriber station's transmit power is controlled by one or more base stations. Each base station, with which the subscriber station is in communication, measures the received signal strength from the subscriber station. The measured signal strength is compared to a desired signal strength level for that particular subscriber station. A power adjustment command is generated by each base station and sent to the subscriber station on the forward link. In response to the base station power adjustment command, the subscriber station increases or decreases the subscriber station transmit power by a predetermined amount. By this method, a rapid response to a change in the channel is effected and the average system performance is improved. Note that in a typical cellular system, the base stations are not intimately connected and each base station in the system is unaware of the power level at which the other base stations receive the subscriber station's signal.
When a subscriber station is in communication with more than one base station, power adjustment commands are provided from each base station. The subscriber station acts upon these multiple base station power adjustment commands to avoid transmit power levels that may adversely interfere with other subscriber station communications, and yet provide sufficient power to support communication from the subscriber station to at least one of the base stations. This power control mechanism is accomplished by having the subscriber station increase its transmit signal level only if every base station, with which the subscriber station is in communication, requests an increase in power level. The subscriber station decreases the subscriber station's transmit signal level if any base station, with which the subscriber station is in communication, requests that the power be decreased. A system for base station and subscriber station power control is disclosed in U.S. Pat. No. 5,056,109 entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM,” issued Oct. 8, 1991, assigned to the assignee of the present invention.
In addition to the above-described reverse link power control, it is also desirable to control the relative power used in each traffic channel transmitted on a forward link by the base station. To enable such a control, each remote station measures the power of traffic channels received form a base station, generates control information in response, and transmits the control information back to the base station. The primary reason for providing such control is to accommodate the fact that in certain locations the forward link may be unusually disadvantaged. Unless the power being transmitted to the disadvantaged subscriber station is increased, the signal quality may become unacceptable. An example of such a location is a point where the path loss to one or two neighboring base stations is nearly the same as the path loss to the active base station, i.e., a base station communicating with the subscriber station. In such a location, the total interference would be increased three times over the interference seen by a subscriber station at a point relatively close to the active base station. In addition, the interference coming from the neighboring base stations does not fade in unison with the signal from the active base station as would be the case for interference coming from the active base station. A subscriber station in such a situation may require 3 to 4 dB additional signal power from the active base station to achieve adequate performance. At other times, the subscriber station may be located where the signal-to-interference ratio is unusually good. In such a case, the base station could transmit the desired signal using a lower than normal transmitter power, reducing interference to other signals being transmitted by the system.
The above described wireless communication service is an example of a point-to-point communication service. In contrast, a point-to-multipoint service is a service where the information transmitted by a source of the information is intended for a plurality of mobile stations. The basic model of a point-to-multipoint system comprises a set of users, a group of which is served by one or more sources of information, which provide information with a certain contents, e.g., news, movies, sports events and the like, to be transmitted to the users. Each user's subscriber station participation in the point-to-multipoint service (a member subscriber station) monitors a forward link shared channel. Because the source of information fixedly determines the content, the users are generally not communicating back. Examples of common usage of such a point-to-multipoint services communication systems are TV broadcast, radio broadcast, and the like. Alternatively, the source of information is a user—a member of the group, which transmits information intended for the remaining members of the selected group. If the user wishes to talk, he presses a push to talk (PTT) button. Typically, the talking user's voice is routed from the subscriber station to a transceiver station on a dedicated reverse link. The transceiver station then transmits the talking user's voice over the forward link shared channel. As in case of the point-to-point communication system, such a communication system allows both landline and wireless subscriber station to access the system. Such a point-to-multipoint service is also referred to as a group service. Examples of the group service communication system use is in dispatch services, such as local police radio systems, taxicab dispatch systems, Federal Bureau of Intelligence and secret service operations, and general military communication systems.
The above-mentioned point-to-multipoint service communication systems are generally highly specialized purpose-build communication systems. With the recent, advancements in wireless cellular telephone systems there has been an interest of utilizing the existing infrastructure of the—mainly point-to-point cellular telephone systems—for point-to-multipoint services. As used herein, the term “cellular” system encompasses system operating on both cellular and personal communication system (PCS) frequencies.
The power control mechanism for subscriber stations acting as point-to-point units described above is not directly applicable to point-to-multipoint services. As discussed, the wireless cellular telephone systems assign a dedicated forward and reverse link between two or more communicating users. In contrast, the point-to-multipoint services typically rely on assigning a shared forward link to be monitored by all the users in the group. Furthermore, in a point-to-multipoint services, in general, majority of the subscriber stations are passive (i.e. just listening) at any one time. When a subscriber station is passive, it does not necessarily have an established reverse link on which to transmit information to the base station. Because the power control method in the existing infrastructure is based on a point-to-point communications model, there is a need in the art for a method and apparatus for a power control that enables group services in an existing infrastructure of a wireless cellular telephone system.